(1) Field of the Invention
The present invention relates to a method for positioning a station by using the time difference of arrival of radio waves in a wireless communication system.
(2) Description of the Related Art
In a method for sensing the position of a mobile terminal station by using the time difference of arrival of radio signals, an error contained in the result of positioning the station is increased disadvantageously unless the time difference of arrival of the signals at individual access points (wireless base stations) is measured as precisely as possible.
In an ideal environment, a signal reception timing can be estimated with high accuracy since the waveform configuration of a correlation value used to measure the time difference of arrival of signals is close to that of a theoretical value. In an actual situation, however, an error factor such as attenuation, thermal noise, or multipath is caused by ambient conditions including a geographical environment, a temperature variation, and a surrounding radio-wave environment and the configuration of the correlation waveform of a received signal is distorted so that there are cases where the reception timing is estimated erroneously. In particular, the multipath is a major factor causing the degradation of positioning accuracy since it is difficult to analytically respond to a dynamic change in the geographical environment.
As an example of a technology for improving range finding accuracy when the multipath is a major error factor in a range finding process, there is a method disclosed in Japanese Unexamined Patent Publication No. HEI 10-246764 (Japanese Patent No. 3127220: Patent Document 1). According to the invention disclosed in Patent Document 1, GPS positioning is performed at each of auxiliary observation points provided in the vicinity of two observation points to allow accurate measurement of the distance between two points by using a GPS, respective distance vectors are determined from a plurality of results of positioning that have been obtained, and the plurality of results are averaged, whereby the influence of the multipath is reduced and accurate measurement of the distance between the two points is enabled.
A description will be given to the method with reference to FIG. 16. In the measurement of the distance between observing points A and B, auxiliary observing points B1, B2, and B3 are placed in the vicinity of the observing point B and the respective auxiliary vectors b1, b2, and b3 thereof are predetermined. Likewise, auxiliary observing points A1, A2, and A3 are also placed in the vicinity of the observing point A and GPS positioning is performed at each of the observing points. By using the results of positioning thus obtained and the foregoing auxiliary vectors b1, b2, and b3, provisional baseline vectors aib1, aib2, and aib3 (i=1 to 3) extending from the observing point A to the observing point B are calculated and the results are averaged, whereby a baseline vector extending from the observing point A to the observing point B is determined.
In a method disclosed in Japanese Unexamined Patent Publication No. 2001-311767 (Patent Document 2), it is determined whether each of signals received from GPS satellites is a direct wave or a reflected wave and positioning calculation is performed by using only the GPS satellite from which the signal determined to be a direct signal has been received. A description will be given to a method for determining a direct wave or a reflected wave. A positioning system is composed of a first antenna and a second antenna with directionality. Information on a GPS satellite A is acquired by using the first antenna and the second antenna is oriented in the direction of the captured GPS satellite. If a signal from the GPS satellite A captured by the first antenna is receivable by the second antenna, the signal is determined to be a direct wave. If the second antenna can not receive the signal from the GPS satellite A, it is determined to be a reflected wave.
In Japanese Unexamined Patent Publication No. 2001-147261 (Patent document 3), a distance is sensed by performing communication between two communication stations by using signals at different frequencies, calculating the respective arrival times thereof, performing the determination of a direct wave or a reflected wave from the results, and selecting the direct wave.